The present invention relates to a gear, such as a differential ring gear in a vehicle drive train system, that is used for transmitting such high torque as to produce a stress not only in a surface layer portion but also in a core portion, and to a manufacturing method of the gear. More in detail, the present invention relates to a gear that is made of steel as raw material and is hardened through processes such as carburizing and quenching, and to a manufacturing method of the gear.